1. Field of the Invention
The present invention relates to a semiconductor display device constituted by using a semiconductor film having a crystalline structure, and more specifically, the invention relates to a semiconductor display device using a crystalline semiconductor film obtained through crystal growth on an insulation surface, and using a field effect transistor, in particular, a thin film transistor. The present invention also relates to a method for manufacturing the semiconductor device.
2. Description of the Related Art
In recent years, a technique of forming a TFT on a substrate has greatly progressed, and its application and development for active matrix semiconductor display devices have been advanced. In particular, since a TFT using a polycrystalline semiconductor film has higher field-effect mobility (also referred to as mobility) than a TFT using a conventional amorphous semiconductor film, it enables high-speed operations. It is therefore possible to control a pixel by a driver circuit formed on the same substrate where the pixel is formed, though the pixel is conventionally controlled by a driver circuit provided outside the substrate.
Incidentally, for substrates used in semiconductor devices, a glass substrate is regarded as being promising in comparison with a single crystal silicon substrate in terms of the cost. A glass substrate is inferior in heat resistance and is easily subjected to thermal deformation. Accordingly, in order to avoid thermal deformation of the glass substrate, in the case where a polysilicon TFT is formed on the glass substrate, the use of laser annealing for crystallization of the semiconductor film is extremely effective.
The laser annealing has characteristics such as remarkable reduction of processing time compared to an annealing method utilizing radiant heating or thermal conductive heating. In addition, a semiconductor film is selectively and locally heated so that a substrate is scarcely thermally damaged.
Note that the term “laser annealing method” herein indicates a technique for re-crystallizing a damaged layer formed in a semiconductor substrate or in a semiconductor film, and a technique for crystallizing a semiconductor film formed on a substrate, for example. This also includes a technique that is applied to planarizing or improvement of a surface quality of the semiconductor substrate or the semiconductor film. Applicable laser oscillation apparatuses are: gas laser oscillation apparatuses represented by an excimer laser; and solid laser oscillation apparatuses represented by a YAG laser. It is known that such a device performs crystallization by heating a surface layer of the semiconductor by irradiation of the laser light in an extremely short period of time of about several tens of nanoseconds to several tens of microseconds.
Laser light is classified into two types: pulse oscillation and continuous oscillation, according to an oscillation method. In the pulse oscillation laser, an output energy is relatively high, so that mass productivity can be increased by setting the size of a beam spot to several cm2 or more. In particular, when the shape of the beam spot is processed using an optical system and made to be a linear shape of 10 cm or more in length, it is possible to efficiently perform irradiation of the laser light to the substrate and further enhance the mass productivity. Thus, for crystallization of the semiconductor film, the use of a pulse oscillation laser is becoming mainstream.
In recent years, however, it has been found that the grain size of crystals formed in a semiconductor film is larger when continuous wave laser is used to crystallize a semiconductor film than when a pulse oscillation laser is used. With crystals of larger grain size in a semiconductor film, the mobility of TFTs formed from this semiconductor film is increased, and the variation in the characteristics of the TFTs caused by grain boundaries is suppressed. As a result, continuous wave laser light is now suddenly attracting attention.
A crystalline semiconductor film manufactured by using a laser annealing method, which is roughly divided into pulse wave and continuous wave types, is generally formed with an aggregation of a plurality of crystal grains. The position and size of the crystal grains are random, and it is difficult to specify the crystal grain position and size when forming a crystalline semiconductor film. Crystal grain interfaces (grain boundaries) therefore exist within an active layer formed by patterning the aforementioned crystalline semiconductor film into an island-like.
Note that the term “grain boundary”, which is also called a crystal grain boundary, refers to one of lattice defects categorized as a plane defect. The plane defect includes not only the grain boundary but also a twin plane, a stacking fault, or the like. In this specification, the plane defects having electrical activity and dangling bonds, i.e., the grain boundary and the stacking fault are collectively called the grain boundary.
In contrast with the crystal grains, countless recombination centers and capture centers exist in the grain boundaries due to an amorphous structure, crystal defects, and the like. It is known that a carrier is trapped in the capture centers, the potential of the grain boundaries rises, and the grain boundaries become barriers with respect to the carrier, and therefore the current transporting characteristics for the carrier are reduced. The existence of grain boundaries within the TFT active layer, in particular within a channel forming region, therefore exerts a great influence on the characteristics of the TFT in which a TFT mobility drops considerably, an ON current is reduced, and an OFF current is increased due to electric current flowing in the grain boundaries. Further, the characteristics of a plurality of TFTs, manufactured on the premise that the same characteristics can be obtained, may vary depending on the existence of grain boundaries within the active layers.
The position and size of the crystal grains obtained when irradiating laser light to a semiconductor film become random due to the following reasons. A certain amount of time is required until the generation of solid state nuclei within a liquid semiconductor film which is completely melted by the irradiation of laser light. Countless crystal nuclei are generated in completely melted regions along with the passage of time, and crystal grows from each of the crystal nuclei. The positions at which the crystal nuclei are generated are random, and therefore the crystal nuclei are distributed non-uniformly. Crystal growth stops at points where the crystal nuclei run into each other, and therefore the position and the size of the crystal grains become random.
It is ideal to form the channel forming region, which exerts a great influence on the TFT characteristics, by a single crystal grain, thus eliminating the influence of grain boundaries. However, it is almost impossible to form an amorphous silicon film, in which grain boundaries do not exist, by using a laser annealing method. It has therefore not been possible to date to obtain characteristics equivalent to those of a MOS transistor, which is manufactured on a single crystal silicon substrate, in a TFT that uses a crystalline silicon film crystallized by employing laser annealing.